Purpose: In previous studies we demonstrated that CTLA4Ig suppresses de novo T-cell activation through blocking CD28 co-stimulation and attenuates recall alloantibody responses following re-exposure to HLA-A2 skin grafts. Here, we investigated the immune mechanisms associated with recall alloantibody suppression by CTLA4Ig.

Methods: A mouse model of HLA.A2 re-sensitization was utilized to investigate the mechanism(s) responsible for suppression of recall donor specific antibody responses by CTLA4Ig. Abatacept (400ug/mouse/day by I.P injection) was given to the mice at days 0, 2, and 7 after re-sensitization with a HLA.A2+ skin graft. Spleens and the bone marrows were harvested at days 7, 10 and 14 post re-sensitization. T-cells, B-cells and plasma cells were studied in multiparameter FACS and digitalized immunofluorescent microscopy.

Results: CD138⁺ plasma cells isolated from the spleens of control mice produced significantly higher levels of DSA IgG than that of CD138⁺ plasma cells isolated from the bone marrow, indicating that the splenic plasma cells are highly activated during recall antibody responses. Immunofluorescent microscopy showed that a significant increase in CD138⁺ plasma cells in the peri arteriolar sheet (PALS) of the spleens in the control mice at Day 6 after re-sensitization, indicating migration of PCs from the blood during recall alloantibody responses. Abatacept treatment, however, was able to reduce CD138⁺ plasma cells proliferating in the spleens. Plasma cell reduction by abatacept was associated with significantly suppression of blood DSA IgG levels in treated allograft recipients (n=4, 88+46 MFI, mean fluorescent intensity), as compared to control mice (n=4, 337+172 MFI, p=0.031). Furthermore, in vitro experiments demonstrated that abatacept inhibited DSA IgG secretion by CD138⁺ plasma cells isolated from the spleens of allograft recipients. Additional experiments using a IgG1 secreting mouse hybridoma cell line showed that abatacept binds to CD80 expressed on these cells with subsequent inhibition of cell proliferation and reduction in IgG ELISpot formation.

Conclusion: Our data suggest suppression of recall DSA responses is associated with a direct suppressive effect by abatacept on B and plasma cells. CTLA4Ig may affect plasma cell function by interrupting CD28-B7 pathway that plays a regulatory role in plasma cell development and subsequent antibody secretion.

CITATION INFORMATION: Wu G, Kim I, Chai N.-N, Jordan S, Klein A. CTLA4Ig Suppresses Memory Responses to HLA-A2⁺ Skin Grafts in a Mouse Model of Allosensitization by Reducing CD138⁺ Plasma Cells. Am J Transplant. 2017;17 (suppl 3).

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